Cell Size There are 2 things that limits cell size: - DNA overload - Exchanging materials

1. DNA Overload A. Most cells contain only one NUCLEUS which controls the cell. B. If cell gets TOO LARGE in size then the nucleus/DNA cannot meet the needs of the growing cell any longer.

DNA overload The larger a cell becomes, the more demands the cell puts on its DNA. When the cell is small, the information in the DNA is enough to meet all the cell’s needs. A growing cell does not make more DNA In time, the cell’s DNA would no longer be able to serve the increasing needs of the cell.

Exchanging materials: I. Surface Area to Volume Ratio What materials do cells exchange through their cell membranes? food water oxygen wastes

b. The rate at which the exchange through the cell membrane takes place depends on: SURFACE AREA

c. The rate at which food & oxygen are produced depends on: THE CELL’S VOLUME

The relationship of surface area to volume creates problems for cell growth: d. Volume increases much more rapidly than surface area.

e. If a cell doubles in size, it would require 8x more nutrients and have 8x more waste, but only 4x more surface area for these substances to travel through.

II. Diffusion A. Diffusion is fast and efficient when the cell is small. B. Diffusion is slow and inefficient when the cell is large.

What is the solution to cells getting bigger? Cell division - forming 2 daughter cells.

Surface area vs. volume

Question What is the surface area Volume Ratio of surface area to volume of a 5cm cube Surface area length X width X no of sides Length X width X height

The cell replicates its DNA.

3. Why do cells divide? 1. Unicellular Organisms – to REPRODUCE 2. Multicellular – for GROWTH, REPAIR, and MAINTENANCE

3. Some cells do not divide: A. NERVE cells B. Only special regions in PLANT cells. C. Only special regions in BONE cells divide – the growth plate D. RED BLOOD cells have NO NUCLEUS, so they do not divide.

What are chromosomes? 1. Structures that contain DNA and become darkly colored when stained. 2. They are the carriers of HEREDITY material. 3. Exist as CHROMATIN, strand of DNA wrapped around PROTEIN molecule. 4. Chromatin is called CHROMOSOMES when it has been replicated, coiled, and condensed so that it can be seen with a MICROSCOPE during mitosis.

What is the cell cycle? 1. A period of GROWTH and DIVISION in cells. 2. Interphase – growth part 3. Cell Division or M-phase – cell division part